Display devices such as a liquid crystal display device include a display device integrated with a touch panel. Such a display device has been developed and has a touch panel (area sensor) function that can detect a touched position on a surface of the touch panel when the surface is touched by an input pen.
A resistive film type (a system in which an input position is detected by contact between an upper conductive substrate and a lower conductive substrate due to a press) and an electrostatic capacitive type (a system in which an input position is detected by detection of capacitance change in a touched position) are typical types of conventional display devices integrated with a touch panel.
Recently, there has been developed a liquid crystal display device in which an optical sensor element such as a photodiode or a phototransistor is provided in each pixel (or each group of a plurality of pixels) in an image display region (for example, see Patent Literature 1). It is thus possible to achieve a function as an area sensor (specifically, a scanner function, a touch panel function and the like) in a regular liquid crystal display device, by providing an optical sensor element in each pixel. That is, it is possible to achieve a display device integrated with a touch panel (or scanner), by causing the optical sensor element to serve as an area sensor.